I've done a bit of research on usb cables and it appears to be the one topic that starts forum wars between those who have actually bought cables and heard an improvement and those who are adamant that digital 0's and 1's aren't affected by a cable other than if the cable is defective.
however general consensus between those who have actually tried a premium usb cable is that they do improve the sound quality and it appears to be from people who have no affiliation with usb cable producers.
here's a handy link. it appears that the Wireworld starlight is one of the most popular choices. Flat cable and looks very slick. There's a cheaper Ultraviolet variety, but people who have used both seem to say that the starlight is a significant upgrade.
http://www.computeraudiophile.com/f8-general-forum/poll-what-usb-cable-are-you-using-7789/index4.html
http://www.addictedtoaudio.com.au/Wireworld-Ultraviolet-USB-1M
http://www.addictedtoaudio.com.au/Wireworld-Ultraviolet-USB-1M-1
I've been bugging Guillaume of A2A to stock the 0.5m model for the last day or two and when he finally does, I've decided to go for the 1m. He must be livid.
As for the Power cable, I'm also trying to get him to stock the entry-level wireworld stratus and aurora. They seem like great value for the price and it can't hurt to try at such a low asking price.
http://www.whathifi.com/Review/Wireworld-Stratus-52/
http://www.audioaficionado.org/general-audio-discussion/14063-best-budget-power-cord.html